1. Field of the Invention
The invention relates to an internal jolting device according to the preamble of patent claim 1.
2. Description of the Related Art
Internal jolting devices, also designated as internal jolters or internal vibrators, are generally known and serve for the compacting of liquid concrete. For this purpose, a jolting cylinder, in which an electric motor and an unbalanced mass driven by the latter are accommodated, is fastened to the end of a relatively long or else relatively short tube in a way corresponding to a tube-type or bar-type jolter. The unbalanced mass rotating at a very high rotational speed generates an oscillation which is coordinated with the concrete to be compacted and is transmitted to the fresh concrete after the jolting cylinder has penetrated into the latter, with the result that air inclusions and pore formation associated with these are eliminated and therefore the bulk density of the concrete is increased, so that the desired quality and strength can be achieved. Appliances of this type have proved extremely successful in practice.
Furthermore, internal jolters based on a similar principle of construction are known, in which a relatively large unbalanced mass is driven at relatively low rotational speed, with the result that a higher oscillation amplitude of the jolting cylinder can be generated. Appliances of this type are suitable less for the compaction of concrete than for the distribution of concrete.
So that the work of processing concrete can be carried out optimally on the construction site, it is therefore necessary to have both types of appliance, thus requiring not only a high material outlay in terms of available appliances, but also the frequent conversion and connection of different types of appliance.
DE-U-73 16 210 discloses an internal jolter with an electric motor which is arranged in a jolting housing and which drives an unbalanced mass in rotation. The eccentricity of the center of gravity of the unbalanced mass can be changed with respect to the axis of rotation of the unbalanced mass, in such a way that a decrease in rotational speed occurring during a decrease in rotational speed when the jolting housing penetrates into the concrete gives rise at the same time to a reduction in the unbalanced mass, so that the decrease in rotational speed can be directly compensated again. It thereby becomes possible to maintain the internal jolter largely at the same rotational speed during the compaction of concrete.
DD 269 568 A1 discloses an adjustable vibrator, in which, in order to set a maximum and minimum exciting force, two unbalanced masses are arranged on a shaft in such a way that one unbalanced mass is mounted fixedly relative to the shaft and the other unbalanced mass is mounted rotably on the shaft. By a change in direction of rotation, the position of the rotable unbalanced mass changes in relation to the fixed unbalanced mass, thus resulting at the same time in a variation in the exciting forces.
The object on which the invention is based is to specify an internal jolter, by means of which fresh concrete can not only be compacted, but also distributed.
The solution for achieving the object is specified in patent claim 1. Advantageous developments of the invention may be gathered from the dependent claims.
By the variation in the eccentricity of a center of gravity of the unbalanced mass with respect to an axis of rotation of the unbalanced mass when the mass of the unbalanced mass is invariable as a consequence of construction, the so-called mr value (the product of the mass and the radius of the center of gravity), which is critical for the oscillation amplitude, can also be varied. At a low mr value, the amplitude is also low, this being preeminently suitable for the compaction of fresh concrete. However, when the mr value rises as a result of a variation in the eccentricity of the center of gravity, the oscillation amplitude also rises, and consequently the proper motion of the jolting cylinder in the fresh concrete. The concrete is thereby less compacted than it is pushed and consequently can easily be distributed in the formwork.
In a preferred embodiment of the invention, the eccentricity can be varied between at least two fixed values, one value being particularly suitable for the compacting work and another value for the distributing work. It is particularly advantageous if the rotational speed of the electric motor and consequently the rotational speed of the unbalanced mass are variable. Consequently, each eccentricity of the center of gravity can be assigned an optimum rotary frequency which is obtained, inter alia, due to the action of the jolting cylinder in the fresh concrete. Suitable values for this purpose can be determined within the framework of simple preliminary tests.
According to the invention, the electric motor is fed by a frequency converter which can be changed over in order to generate at least two different electrical frequencies. When the frequency converter is arranged in a switch housing of the internal jolting device, the operating elements necessary for the changeover can also easily be mounted on the switch housing.
According to a particular development of the invention, the direction of rotation of the electric motor can be changed over and the rotary device has a shaft which is coupled to the electric motor and on which two mass elements forming the unbalanced mass are arranged, in such a way that a first mass element is fastened to the shaft and a second mass element is rotable on the shaft relative to the first mass element between two end positions.
The first mass element always corotates with the direction of rotation of the shaft predetermined by the electric motor. When the direction of rotation is reversed, the first mass element therefore immediately follows this reversal in direction of rotation. The second mass element which freely rotates along the shaft within specific limits, to be precise the two end positions, remains in the initial position due to its inertia and is therefore rotated relative to the first mass element on the shaft Only when the second end position, which may be defined, for example, via a driver, is reached does the second mass element also follow the then reversed direction of rotation of the first mass element. By means of a suitable arrangement and mass distribution on the mass elements, it is thereby possible to ensure that the overall center of gravity of the unbalanced mass resulting from the two mass elements has different eccentricities and therefore different mr values in the two end positions.